Generally, an audio system includes an audio source, an audio power amplifier and a loudspeaker. The output power of the audio system determines the loudness of the system, namely how loud the sound is. The audio source is where the sound originates and may be divided as a digital audio source or an analog audio source according to different storage modes. The audio source for recording and processing analog signals, such as an audio signal propagating in the form of current and voltage, is the analog audio source; the digital audio source records and processes abstract binary data stream formed by permutation and combination of 0 and 1, and the data may be recovered to the analog audio signal by means of the digital to analog converter (DAC). The audio power amplifier is a component for performing power amplification on a weak audio signal to obtain a strong signal. For the audio power amplifier, a main performance index is THD+N, namely Total Harmonic Distortion plus Noise, which is also a condition for determining the rated output power of the audio power amplifier. The loudspeaker, which is also referred to as the horn, is a transducer device to transform an electrical signal into a sound signal. The performance of the loudspeaker greatly affects the tone quality.
In the audio system, with a given audio source, the output power of the audio system is mainly determined by the loudspeaker and the output power of the power amplifier.
In the audio system, the maximum audio power transferred to a specified resistive load is limited by the maximum voltage amplitude output by the audio power amplifier or the maximum amplitude of current which can be transmitted by the audio power amplifier. For most audio power amplifiers, the voltage amplitude greatly depends on the power supply voltage of the amplifier. Therefore, for a specified load, the maximum current and the output power would be determined by the voltage amplitude.
Generally, in the design of an audio amplifier, specific design object and power supply voltage should be taken into account. However, if greater power is desired for the designer of the audio system and there is limitation due to a low power supply voltage, unconventional design method may be needed. Currently, the commonly used methods are: 1) reducing the internal resistance of the loudspeaker; and 2) increasing the operating voltage of the audio power amplifier. Although these two methods can achieve the object of the high power output, there are obvious disadvantages as follows.
1) In the case of reducing the internal resistance of the loudspeaker, the current passing through the loudspeaker may increase. This method thus is limited by the overcurrent capacity of the loudspeaker and achieves limited effect on increasing the power. Since the power loss of the loudspeaker increases with increasing current, the efficiency of this method is low.
2) In the case of increasing the operating voltage of the audio power amplifier, if there is low power supply voltage, the method requires power supply conversion. However, none of the power supply conversions has an efficiency of 100%. Taking the Boost structure which has higher conversion efficiency as an example, its conversion efficiency is only 80%-95%.
Moreover, the operating voltage of the audio power amplifier is limited by the manufacture process and is impossible to increase without limit. Taking the current standard 0.34 μm process as an example, the maximum withstand voltage of a port guaranteed by this process is only 5.5V.
Therefore, the two existing solutions for improving the output power of the audio system are both achieved with efficiency loss and both have certain limitations.